Prof. M. Kostic Mechanical Engineering NORTHERN ILLINOIS UNIVERSITY Sampling and Aliasing:An Interactive and On-Line Virtual Experiment "What We See is Not What It Is!"
Figure 2: The aliasing examples: “wagon wheel illusion,” appearance that a spoke wheel rotates backward while it moves forward (left); a stroboscope experiment to “freeze” rotating fan blades or to make them appear to move slowly forward or backward (center); and appearance of staircase steps along edges (jaggies) in an image (right). What are “sampling” and “aliasing” ?
W.W.S.N.W.I.I."What We See is Not What It Is!" • It’s all about ...… Sampling and Aliasing! • Even ...W.Y.S.W.Y.G."What You See is What You Get!” • …is REALLY NOT true (the same)on different Monitors and Printers
Signal/Sampling Resolution Signal/Magnitude Resolution Sampling/Time Resolution (Speed)
There is NOsuch thing as... • …too large hard drive disk, or ... • …too large sampling resolution! • Well praised ... high-definition digital graphicsand CD-quality music • …are NOT good (true) enough!
(Under)Sampling & Aliasing ... • Under-sampling looses (important) wave details! • But it also may change the wave form/shape due to Aliasing !
Real measurements of a simple sine-harmonic How come this if the measured signal is a simple sine-harmonic wave?
Real signal Sampled signal Sampling-Beat Wave
Real signal Sampled signal Good Sampling
Real signal Sampled signal Sampling with Aliasing
If RPM and Strobe speeds (frequencies) are the same the reference mark will appear stationary (ZERO aliasing) If the Strobe speeds is half of RPM, the disk will turn twice and the mark will appear stationary again! Rotating disk (RPM) Stroboscope light If the Strobe speed is slower If the Strobe speed is faster Sampling RPM with Stroboscope
ZERO Aliasing NO Alias. Aliasing Folding Diagram
Sampling and Aliasing… Sampling is a discrete observation or measurement, while Aliasing is an illusion, appearance of something that is NOT, due to shortcomings of sampling, …thus everything we perceive or measure is, to a lesser or grater degree, an ILLUSION, responsible for many discoveries and re-discoveries in invention history.
Sampling and Aliasing… Even if we are not interested in those high-frequency signal components, the problem is that they may “fold-back” and falsely appear as low-frequency components that do not exist in reality at all, thus trick us. Such phenomena are well-known as aliasing, which is actually an illusion.
Interactive Experiments ... LabVIEW Virtual Instruments Internet Interactive Experiment Animated Movie
Real signal Sampled signal The Art of Sampling and Aliasing PHASE Shift MAGNITUDE Ratio
Conclusion… This interactive module will stimulate users’ curiosity and motivate them to conveniently “check out” different options and possibilities, and thus accelerate experience and by active, what-if inquiry and “experimentations” enhance their comprehension and interest for deeper thinking and understanding…
Conclusion… These new learning tools are not and cannot replace the traditional learning by cognitive thinking, but, if designed well, the new simulation tools may qualitatively enhance learning environment by stimulating inquiry and building confidence and motivation – in short, may become a “virtual eye and mind opener”!.
NO SPEED LIMIT No Limits … The Future Belongs To… … Whoever Gets There First
Prof. M. Kostic Mechanical Engineering NORTHERN ILLINOIS UNIVERSITY You may contact Prof. Kostic at: mailto:email@example.com or on the Web:www.kostic.niu.edu http://prof.mkostic.com